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Using ABAC Authorization

Attribute-based access control (ABAC) defines an access control paradigm whereby access rights are granted to users through the use of policies which combine attributes together.

• Policy File Format
• Authorization Algorithm
• Kubectl
• Examples
• A quick note on service accounts

Policy File Format

For mode ABAC, also specify --authorization-policy-file=SOME_FILENAME.

The file format is one JSON object per line. There should be no enclosing list or map, just one map per line.

Each line is a “policy object”. A policy object is a map with the following properties:

• Versioning properties:
  • apiVersion, type string; valid values are “abac.authorization.kubernetes.io/v1beta1”. Allows versioning and conversion of the policy format.
  • kind, type string: valid values are “Policy”. Allows versioning and conversion of the policy format.
• spec property set to a map with the following properties:
  • Subject-matching properties:
    • user, type string; the user-string from --token-auth-file. If you specify user, it must match the username of the authenticated user.
    • group, type string; if you specify group, it must match one of the groups of the authenticated user. system:authenticated matches all authenticated requests. system:unauthenticated matches all unauthenticated requests.
  • Resource-matching properties:
    • apiGroup, type string; an API group.
    • Ex: extensions
    • Wildcard: * matches all API groups.
    • namespace, type string; a namespace.
    • Ex: kube-system
    • Wildcard: * matches all resource requests.
    • resource, type string; a resource type
    • Ex: pods
    • Wildcard: * matches all resource requests.
  • Non-resource-matching properties:
    • nonResourcePath, type string; non-resource request paths.
    • Ex: /version or /apis
    • Wildcard:
      • * matches all non-resource requests.
      • /foo/* matches all subpaths of /foo/.
  • readonly, type boolean, when true, means that the Resource-matching policy only applies to get, list, and watch operations, Non-resource-matching policy only applies to get operation.

NOTES: An unset property is the same as a property set to the zero value for its type (e.g. empty string, 0, false). However, unset should be preferred for readability.

In the future, policies may be expressed in a JSON format, and managed via a REST interface.

Authorization Algorithm

A request has attributes which correspond to the properties of a policy object.

When a request is received, the attributes are determined. Unknown attributes are set to the zero value of its type (e.g. empty string, 0, false).

A property set to "*" will match any value of the corresponding attribute.

The tuple of attributes is checked for a match against every policy in the policy file. If at least one line matches the request attributes, then the request is authorized (but may fail later validation).

To permit any authenticated user to do something, write a policy with the group property set to "system:authenticated".

To permit any unauthenticated user to do something, write a policy with the group property set to "system:unauthenticated".

To permit a user to do anything, write a policy with the apiGroup, namespace, resource, and nonResourcePath properties set to "*".

Kubectl

Kubectl uses the /api and /apis endpoints of api-server to negotiate client/server versions. To validate objects sent to the API by create/update operations, kubectl queries certain swagger resources. For API version v1 those would be /swaggerapi/api/v1 & /swaggerapi/experimental/v1.

When using ABAC authorization, those special resources have to be explicitly exposed via the nonResourcePath property in a policy (see examples below):

• /api, /api/*, /apis, and /apis/* for API version negotiation.
• /version for retrieving the server version via kubectl version.
• /swaggerapi/* for create/update operations.

To inspect the HTTP calls involved in a specific kubectl operation you can turn up the verbosity:

kubectl --v=8 version

Examples

1. Alice can do anything to all resources:

   {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "alice", "namespace": "*", "resource": "*", "apiGroup": "*"}}

2. Kubelet can read any pods:

   {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "kubelet", "namespace": "*", "resource": "pods", "readonly": true}}

3. Kubelet can read and write events:

   {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "kubelet", "namespace": "*", "resource": "events"}}

4. Bob can just read pods in namespace “projectCaribou”:

   {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"user": "bob", "namespace": "projectCaribou", "resource": "pods", "readonly": true}}

5. Anyone can make read-only requests to all non-resource paths:

   {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"group": "system:authenticated", "readonly": true, "nonResourcePath": "*"}}
   {"apiVersion": "abac.authorization.kubernetes.io/v1beta1", "kind": "Policy", "spec": {"group": "system:unauthenticated", "readonly": true, "nonResourcePath": "*"}}

Complete file example

A quick note on service accounts

A service account automatically generates a user. The user’s name is generated according to the naming convention:

system:serviceaccount:<namespace>:<serviceaccountname>

Creating a new namespace also causes a new service account to be created, of this form:

system:serviceaccount:<namespace>:default

For example, if you wanted to grant the default service account in the kube-system full privilege to the API, you would add this line to your policy file:

{"apiVersion":"abac.authorization.kubernetes.io/v1beta1","kind":"Policy","spec":{"user":"system:serviceaccount:kube-system:default","namespace":"*","resource":"*","apiGroup":"*"}}

The apiserver will need to be restarted to pickup the new policy lines.